The present invention relates to the field of electrical protection circuitry, and more particularly to removable ESD shunts for protection of transducing heads against electrostatic discharge and electrical overstress during fabrication and testing.
Hard disc drive (HDD) systems are widely used for data storage and retrieval. A HDD system typically includes a magnetic storage medium, such as a rotatable magnetic disc, and a read/write transducing head. In operation, the magnetic disc rotates while the read/write transducing head, supported by a slider, “flies” above a surface of the rotating disc on a small cushion of air located at and air bearing surface (ABS) of the transducing head. The transducing head and slider are positioned relative to the disc by a spring-loaded suspension attached to an actuator arm. The small cushion of air works in conjunction with spring bias force from the suspension to maintain the transducing head at a substantially constant distance from the rotating disc, generally at a distance of about 10 nanometers.
The transducing head typically includes a reader and a writer. Currents and voltages generally do not damage the writer. The reader, however, includes a read element, such as a magnetoresistive (MR) sensor element comprising a number of layers, which is highly sensitive to electrostatic discharge (ESD) and electrical overstress (EOS). ESD specifically refers to an actual discharge, whereas EOS refers to a voltage or current in the element larger than that intended under normal operating conditions; however, as used hereinafter, the terms are intended to contain each other.
Many varieties of MR sensor elements are known, but all MR sensor elements have relatively small physical sizes, allowing use of the MR sensor element with magnetic discs having a greater areal density of recorded data. For example, an MR sensor element used for high recording densities will have a cross-section of about 500 Angstroms (Å) by 0.1 micrometers (μm) or smaller. The small size of the MR sensor element makes the MR sensor element particularly susceptible to damage from ESD or EOS. Discharges having a voltage of less than a volt through such a physically small MR sensor element are capable of damaging or destroying the MR sensor element. Numerous types of damage to the MR sensor element are possible, including: destruction of the MR sensor element by thermal breakdown due to melting, evaporation, or diffusion of the layers of the MR sensor element; contamination of an air bearing surface of the transducing head; the creation of shorts by electrical breakdown; and other forms of transducing head performance degradation.
ESD occurs, for example, when charged objects, equipment, or persons contact or nearly contact the transducing head, causing triboelectric charging of the transducing head. Materials such as plastics can cause significant charge accumulation, which can destroy a transducing head upon a discharge between materials and the transducing head.
ESD and EOS are particularly problematic during fabrication of the HDD. Transducing heads are initially fabricated as part of a wafer that contains, for example, about 20,000 transducing heads. Various manufacturing processes are performed on the transducing heads during wafer-level fabrication. Individual transducing heads are then separated from the wafer and mounted on a suspension. The suspension is assembled in a head stack assembly (HSA), which is then assembled in the HDD.
It is desired to test a transducing head at various stages of HDD fabrication. Testing allows detection of defective transducing heads prior to assembly in an HDD, thereby reducing waste and expense otherwise generated when additional manufacturing processes are performed on assemblies that include a defective transducing head.
Klaassen, U.S. Pat. No. 6,400,534, discloses a magnetoresistive head assembly with a shunt system, whereby standard guarded measurements of a transducing head are taken to test the MR sensor element at various stages of the manufacturing process. However, the shunt is not integrated with the transducing head and is not electrically removable without damage to the transducing head.
Phipps et al., U.S. Pat. No. 5,638,237, discloses a removable shunt system. The shunt system does not disclose testing of a transducing head after the shunt system has been installed.
Moreover, known means for electrical removal of the shunt generate a pulse which can cause damage to the magnetoresistive head. Shunts including two or more fuses generate damaging pulses as the fuses breakdown non-simultaneously.
Other designs are known where external shunting circuitry, such as switches or other large circuitry, is connected to the transducing head. Those external circuitry designs have a number of disadvantages, including inefficiencies of space, cost, reliability, and performance.
It is also possible to test the entire transducing head with the shunt in place and without employing guarded measurements. However, known shunts do not have constant and precisely known resistances during fabrication, thereby frustrating attempts to accurately calculate a resistive effect of the sensor element by backing out a resistive effect of the shunt from measurements of the entire transducing head. Stages of fabrication of an HDD often occur in a variety of factory locations, which have varying environmental conditions. Specifically, environmental temperature may vary during fabrication of the HDD. For example, wafers containing numerous transducing heads may be fabricated in a facility different from a facility used to separate wafers into individual transducing heads (i.e. sliders) and a facility used to attach transducing heads to a head gimbal assembly (HGA). Temperature conditions may vary between the various facilities.
Temperature conditions affect a resistance of a shunt having a temperature coefficient of resistance (TCR) not equal to zero. Changes in the resistance of the shunt are typically unpredictable. Thus, attempts to measure a resistance of the sensor element by measuring the resistance of the entire transducing head and adjusting the measurement to exclude the resistance of the shunt are limited by the changing resistivity of the shunt. Proxy measurements of resistances of separate test structures or toolkit shunts not installed in the transducing head introduce errors into measurement, due to such factors as varying resistances of the fuses due to manufacturing tolerances.
Thus, a novel design is needed to provide a transducing head with an ESD shunt that may be repeatedly tested around during fabrication of an HDD and also is permanently removable prior to operation of the transducing head.